Don't throw in Zotero.DataObjects::get() for nonexistent objects

Return false for single ids or skip for multiple ids. This is the original
behavior, but at some point it started throwing an UnloadedDataException. IDs
are always loaded at initialization, though, so we know whether the objects
actually exist.

test/tests/dataObjectsTest.js

@@ -3,6 +3,12 @@
 describe("Zotero.DataObjects", function () {
 	var types = ['collection', 'item', 'search'];
 	
+	describe("#get()", function () {
+		it("should return false for nonexistent objects", function* () {
+			assert.isFalse(Zotero.Items.get(3464363));
+		});
+	});
+	
 	describe("#getLibraryAndKeyFromID()", function () {
 		it("should return a libraryID and key within a transaction", function* () {
 			for (let type of types) {